Magnesium-based hydrogen storage alloys

ABSTRACT

Magnesium-based hydrogen storage alloys comprise a metallic magnesium (Mg) and a magnesium-containing intermetallic compound (Mg x M y  wherein y is 1-x) and contain not less than 60 mass % of magnesium in total, and have a phase of a primarily crystallized magnesium-containing intermetallic compound in its solidification structure.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to hydrogen storage alloys useful as amaterial for transportation and storage of hydrogen, and moreparticularly to magnesium-based hydrogen storage alloys having a largehydrogen storage quantity at a lower temperature under a lower pressure.

[0003] 2. Description of Related Art

[0004] Recently, a hydrogen energy is expected as a petroleumalternative energy. However, there are left many problems to be solvedsuch as infrastructure building, safety and the like for putting thehydrogen energy into practice.

[0005] When hydrogen is filled into a high-pressure steel bottle, avolume can be compressed to about {fraction (1/150)} or to about{fraction (1/800)} at a liquefied state. On the other hand, hydrogenstorage alloys can occlude (absorb) a gaseous hydrogen therein and storeas a solid, so that a volume of hydrogen can be apparently compressed toabout {fraction (1/1000)}. Furthermore, the hydrogen stored in thehydrogen storage alloys is simple in the handling as compared with aliquid hydrogen or a high pressure hydrogen gas. Therefore, there is agreat merit in practical use when hydrogen is transported and stored byusing the hydrogen storage alloys, and it is advantageous in the safetybecause it is not required to handle hydrogen as a high pressure gas ora liquid.

[0006] And also, hydrogen can be absorbed or desorbed only by adjustinga temperature or a pressure in the hydrogen storage alloys, so that itis possible to build a cheap hydrogen storage equipment by using thehydrogen storage alloys and it is attempted to reduce energy cost.

[0007] From the above reasons, or from a viewpoint of a future energyagenda, it is urgently demanded to develop hydrogen storage alloys fortransporting and storing hydrogen in a high performance.

[0008] As the conventionally discovered hydrogen storage alloys, thereare mainly known binary intermetallic compounds of AB₅ type such asLaNi₅ or the like, AB₂ type such as ZrMn₂ or the like, AB type such asTiFe or the like, and A₂B type such as Mg₂Ni or the like. Among them, ahydrogen storage quantity of the hydrogen storage alloys other thanMg₂Ni is as small as about 1.4 mass % in LaNi₅, about 1.7 mass % inZrMn₂, and about 1.8 mass % in TiFe based on the weight of the alloys,which is mainly composed of a relatively heavy element such as a rareearth element or Zr, so that it is difficult to increase the hydrogenstorage quantity per the weight, and V and the like are expensive andare costly less in the merit.

[0009] On the other hand, Mg₂Ni as a typical example of A₂B type alloyis about 3.6 mass % in the hydrogen storage quantity, which isconsiderably larger than that of the other alloys. However, it isdemanded to develop a hydrogen storage alloys with a higher capacity.Particularly, hydrogen storage alloys mainly composed of magnesium areknown to have a very large hydrogen storage performance. Moreover, ahydrogen storage quantity of magnesium metal itself (H₂/(H₂+Mg)) reaches7.6 mass %.

[0010] However, the magnesium-based hydrogen storage alloys are not putinto a practical use up to the present time. Because, it is consideredto be difficult to initially activate the magnesium alloys. That is,magnesium is easy to absorb hydrogen and to form a stable hydride, sothat it is required to maintain at high temperature and high pressurestate of 350-450° C. and 10-20 MPa for absorbing and desorbing hydrogen,which becomes difficult to put into the practical use as a hydrogenstorage alloy.

SUMMARY OF THE INVENTION

[0011] It is, therefore, an object of the invention to providemagnesium-based hydrogen storage alloys capable of easily absorbing anddesorbing a greater quantity of hydrogen at practical conditions (lowtemperature and low pressure).

[0012] The inventors have made various studies for solving the aboveproblems inherent to the conventional magnesium-based hydrogen storagealloys to achieve the above object, and found that there is a constantrelationship between an initial activation property and a metalstructure, and as a result, the invention has been accomplished adding ametallographical consideration.

[0013] That is, the invention,,lies in a magnesium-based hydrogenstorage alloys comprising a metallic magnesium (Mg) and amagnesium-containing intermetallic compound (Mg_(x)M_(y) wherein y is1-x) and containing not less than 60 mass % of magnesium in total, andhaving a phase of a primarily crystallized magnesium-containingintermetallic compound in its solidification structure. In this case, itis favorable that an amount of metallic magnesium occupied in totalmagnesium (≧60 mass %) is about 7-45 mass % and the remainder ismagnesium in the magnesium-containing intermetallic compound.Furthermore, it is favorable that an amount of magnesium in theintermetallic compound is about 16-63 mass %.

[0014] In the invention, an alloying element constituting themagnesium-containing intermetallic compound is preferable to be at leastone element selected from the group consisting of Al, Si, Ca, Co, Ni,Cu, Sr, Y, Pd, Sn, Ba and Ln (lanthanide elements). And also, it isfavorable to be initially activated at a temperature of not higher than300° C. under a hydrogen pressure condition of not more than 3 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a diagram of a Mg—Ni series metal.

[0016]FIG. 2 is a photograph of the metal structure of a Mg—Ni hydrogenstorage alloy according to the invention.

[0017]FIG. 3 is a photograph of metal structure of a Mg—Ni hydrogenstorage alloy according to the comparative example.

DETAILED DESCRIPTION OF THE INVENTION

[0018] As mentioned above, the alloys mainly composed of magnesium arecharacterized by having a large hydrogen storing quantity but has adrawback that an initial activation property capable of absorbing anddesorbing hydrogen is poor.

[0019] This drawback is considered due to the fact that the metallicmagnesium phase itself is poor in the catalytic action through thedissociation of a hydrogen gas molecule and that the diffusion rate ofhydrogen in a magnesium hydride produced on the surface of thehydrogenation is slow, the hydrogenation is not proceeded into theinside of the substrate.

[0020] Based on the above knowledge, the invention improves the initialactivation property by controlling the solidification structure ofmagnesium or magnesium-containing intermetallic compound whilemaintaining a large hydrogen storage property inherent to magnesium. Thethus developed magnesium-based hydrogen storage alloys according to theinvention are magnesium (hereinafter referred to as Mg) as an essentialcomponent and contains Mg-containing intermetallic compound capable ofoccluding hydrogen.

[0021] As an alloying element forming the intermetallic compound bybonding to Mg (hereinafter abbreviated as M), use may be made of atleast one element selected from the group consisting of Ln (lanthanideelements), Ni, Ca, Al, Ba, Cu, Pd, Si, Sr, Y and Sn. These elements forman intermetallic compound of a general formula: Mg_(x)M_(y)equilibrating with a Mg phase.

[0022] In the invention, the Mg_(x)M_(y) intermetallic compound is usedas a hydrogen storage alloy and formed by mixing the Mg_(x)M_(y)intermetallic compound and metallic Mg at a given composition ratio andmelting and casting and alloying them. In this case, a two phasecomprising Mg phase and Mg_(x)M_(y) intermetallic compound phase isproduced accompanied with eutectic reaction from a liquid phase duringthe solidification to form a hydrogen storage alloy, but it is importantto have a composition precipitating a primary crystal of Mg-containingintermetallic compound without initially precipitating Mg during thesolidification.

[0023] In general, Mg phase is precipitated as a primary crystal duringthe solidification in alloys having a Mg content higher than an eutecticpoint, while Mg_(x)M_(y) intermetallic compound phase is precipitated asa primary crystal during the solidification in alloys having a Mgcontent lower than an eutectic point.

[0024] Although a phase diagram of Mg—Ni binary alloy is shown in FIG.1, when a total amount of Mg including metallic Mg of a matrix portionis less than an eutectic point (Mg: 76.5 mass%), for example, within arange of 60-76.4 mass %, a structure as shown in a metallic structurephotograph (×100 magnification) of FIG. 2, i.e. a structure whereinMg_(x)M_(y) intermetallic compound is precipitated as a primary crystalis produced during the solidification. On the other hand, when the totalamount of Mg exceeds 77 mass %, a structure as shown in a metallicstructure photograph (×100 magnification) of FIG. 3, i.e. a structurewherein the metallic Mg pointedly dispersed in the matrix portion (mixedphase of Mg and Mg₂Ni) is precipitated as a primary crystal.

[0025] In the former case (FIG. 2), M element (other than Ni) is easilyactivated in the first hydrogenation of Mg_(x)M_(y) phase to decomposeinto a hydride of the M element or two phases of M element and Mghydride phase to thereby cause dismutation reaction. The hydride of theM element or the M element produced by this dismutation reactionprovides a diffusion (introduction) path of hydrogen in Mg phase andacts as a catalyst dissipating hydrogen gas molecule into monoatom. Forthis end, the Mg-based hydrogen storage alloy can be easily andinitially activated to easily occlude a great amount of hydrogen.

[0026] Moreover, when the M element is Ni, Mg₂Ni intermetallic compounditself produced is high in the catalytic performance and reacts withhydrogen to directly form Mg₂NiH₄, which forms a hydrogen diffusion pathin the Mg phase, so that these alloys are also easily and initiallyactivated likewise the aforementioned M element.

[0027] In the latter case (FIG. 3), when the Mg phase itself solidifiedas a primary crystal is hydrogenated, a stable hydride is formed on thesurface. As a result, a hydrogen diffusion rate in the Mg hydridebecomes slow and hence hydrogenation does not proceed into the inside,while the Mg phase itself is poor in the catalytic action for thedissipation of hydrogen gas molecule and the hydrogen diffusion path isnot formed in the alloy, so that the activation is considerablydifficult. Therefore, hydrogen can not be occluded to a theoreticalocclusion quantity.

[0028] In the hydrogen storage alloys according to the invention, thetotal amount of Mg including Mg of the intermetallic compound is notless than 60 mass % from a viewpoint of obtaining a theoretical hydrogenocclusion quantity of not less than 5 mass %. The reason is described asfollows. Considering a case that the above Mg intermetallic compound isMg_(x)M_(y), when hydrogen is stored in these alloys to hydrogenateMg₂Ni, there are produced the following reaction formulae (1) and (2):

Mg₂Ni+2H₂→Mg₂NiH₄(4H/Mg₂Ni=3.7 mass %)   (1)

Mg+H₂→MgH2(2H/Mg=8.2 mass %)   (2)

[0029] As seen from the formulae (1) and (2), when Mg₂Ni and Mg arehydrogenated, 2 mol of hydrogen atom bonds to Mg/mol irrespectively ofthe form difference. That is, when Mg is 100 mass %, the hydrogenocclusion quantity is 8.2 mass %, so that in order to ensure a hydrogenocclusion quantity of 5.0 mass %, Mg content is required to be not lessthan 60 mass % (5.0 mass %/8.2 mass %×100≅60%).

[0030] On the other hand, the upper limit of the total Mg amount is aneutectic point composition as mentioned above. The upper limit of Mgmass % in each alloying element (M) is shown in Table 1 together with acompound form. TABLE 1 Mg Mg Element content Compound Element contentCompound Ni 77 Mg₂Ni Sr 82 Mg₁₇Sr₂ Ca 84 Mg₂Ca Y 74 Mg₂₄Y₅ Al 68Mg₁₇Al₁₂ La 90 Mg₁₂La Ba 88 Mg₁₇Ba₂ Si 98 Mg₂Si Cu 70 Mg₂Cu Sn 64 Mg₂SnPd 73 Mg₆Pd

EXAMPLE

[0031] A sample used in this example is prepared by adjusting componentsas shown in Table 2, melting them in a high frequency inductionapparatus, and casting in a water-cooling iron mold. Then, the hydrogenstorage alloys prepared for test is pulverized to a size of aboutseveral mesh and weighed by 2 g. The thus weight sample is sufficientlyevacuated at 300° C. and kept at this temperature under a hydrogenpressure of 3 MPa for 24 hours or 120 hours, during which a hydrogenstorage quantity is calculated from a pressure change to evaluate aninitial activation property. The evaluated results are shown in Table 2together with compositions of example alloys and comparative examplealloys. TABLE 2 Activation rate After After Metallic Composition 24hours 120 hours Mg_(x)M_(y) magnesium (mass ratio) (mass %) (mass %)(mass %) (mass %) Example 1 Mg₆₅Al₃₅ 4.4 5.0 Mg 45 Al 35 20 Example 2Mg₇₀Si₃₀ 4.1 5.1 Mg 52 Si 30 18 Example 3 Mg₇₀Ca₃₀ 4.2 5.1 Mg 36 Ca 3034 Example 4 Mg₇₀Ni₃₀ 4.9 5.3 Mg 25 Ni 30 45 Example 5 Mg₆₀Cu₄₀ 4.0 5.0Mg 31 Cu 40 29 Example 6 Mg₇₅Sr₂₅ 4.2 5.3 Mg 59 Sr 25 16 Example 7Mg₇₀Y₃₀ 4.7 5.2 Mg 39 Y 30 31 Example 8 Mg₆₅Pd₃₅ 4.6 5.1 Mg 48 Pd 35 17Example 9 Mg₆₀Sn₄₀ 4.0 5.0 Mg 16 Sn 40 44 Example 10 Mg₇₀Ba₃₀ 4.1 5.0 Mg45 Ba 30 25 Example 11 Mg₇₀La₃₀ 4.3 5.0 Mg 63 La 30 7 Example 12Mg₇₀Ni₂₀Ca₁₀ 4.6 5.2 Mg 29 Ni 20 41 Ca 10 Example 13 Mg₇₀Ni₁₀Ca₁₀La₁₀4.5 5.1 Mg 41 Ni 10 29 Ca 10 La 10 Comparative Mg 0.1 0.7 Mg 100 — — 100Example 1 Comparative Mg₇₈Ni₂₂ 0.2 1.0 Mg 18 Ni 22 60 Example 2

[0032] From the above test results, it has been confirmed that the Mgintermetallic compound is precipitated as a primary crystal in allalloys of Examples 1-13.

[0033] Moreover, in single Mg body of Comparative Example 1 and thealloy of Comparative Example 2 having a composition of solidifying Mg asa primary crystal, the theoretical hydrogen occlusion quantity is largebecause the hydrogen occlusion quantity is determined by the Mg content,but they can not be activated under low temperature and low pressureconditions of 300° C. and 3 MPa, so that the hydrogen occlusion quantityafter 120 hours is small. On the other hand, in the alloys of Examples1-13, it is possible to occlude hydrogen up to substantially anequilibrium state after 120 hours under the above test conditions (300°C., 3 MPa). As a result, it can be seen that the alloys according to theinvention can be sufficiently activated even under practical conditionswhile maintaining the high hydrogen storage performance of Mg-basedalloys.

[0034] As mentioned above, according to the invention, there can beprovided Mg-based hydrogen storage alloys having a high hydrogen storageproperty suitable for transporting and storing hydrogen. Furthermore,the invention facilitates the occlusion and discharge of hydrogen underpractical environment, so that it contributes to minimize machines andequipments using such alloys and to decrease the cost.

What is claimed is:
 1. Magnesium-based hydrogen storage alloyscomprising a metallic magnesium (Mg) and a magnesium-containingintermetallic compound (Mg_(x)M_(y) wherein y is 1-x) and containing notless than 60 mass% of magnesium in total, and having a phase of anprimary crystallized magnesium-containing intermetallic compound in itssolidification structure.
 2. Magnesium-based hydrogen storage alloysaccording to claim 1, wherein an alloying element constituting themagnesium-containing intermetallic compound is at least one elementselected from the group consisting of Al, Si, Ca, Co, Ni, Cu, Sr, Y, Pd,Sn, Ba and Ln (lanthanide elements).
 3. Magnesium-based hydrogen storagealloys according to claim 1 or 2, wherein the alloys are initiallyactivated at a temperature of not higher than 300° C. under a hydrogenpressure condition of not more than 3 MPa.